The present invention relates to electromagnetically actuated traction enhancing differentials, and more particularly to demagnetizing systems therefor.
Differentials are well known in the prior art and allow a pair of output shafts operatively coupled to an input shaft to rotate at different speeds, thereby allowing the wheel associated with each output shaft to maintain traction with the road while the vehicle is turning. Such a device essentially distributes the torque provided by the input shaft between the output shafts. However, the necessity for a differential which limits the differential rotation between the output shafts to provide traction on slippery surfaces is well known.
The completely open differential, i.e. a differential without clutches or springs, is unsuitable in slippery conditions where one wheel experiences a much lower coefficient of friction than the other wheel, for instance, when one wheel of a vehicle is located on a patch of ice and the other wheel is on dry pavement. In such a condition, the wheel experiencing the lower coefficient of friction loses traction and a small amount of torque to that wheel will cause a "spin out" of that wheel. Since the maximum amount of torque which can be developed on the wheel with traction is equal to torque on the wheel without traction, i.e. the slipping wheel, the engine is unable to develop any torque and the wheel with traction is unable to rotate. A number of methods have been developed to limit wheel slippage under such conditions.
Prior methods of limiting slippage between the side gears and the differential casing use a frictional clutch mechanism, either clutch plates or a frustoconical engagement structure, and a bias mechanism, usually a spring, to apply an initial preload between the side gears, and the differential casing. By using a frictional clutch with an initial preload, for example a spring, a minimum amount of torque can always be applied to the wheel having traction, i.e. the wheel located on dry pavement. The initial torque generates gear separating forces which further engage the frictional clutch and develop additional torque. Examples of such limited slip differentials are disclosed in U.S. Pat. Nos. 4,612,825 (Engle), 5,226,861 (Engle) and 5,556,344 (Fox), which are assigned to the assignee of the present invention and are expressly incorporated herein by reference.
The initial preload initiates the development of side gear separating forces which provide further braking action between the side gears and the differential casing. In general, gear separating forces are forces induced on any set of meshing gears by the application of torque to the gears, which forces tend to separate the gears. In a differential, the development of torque will create side gear separating forces which tend to move the side gears away from the pinion gears. When one wheel is on a surface having a low coefficient of friction, the initial preload creates some contact and frictional engagement between the differential casing and the clutch mechanism disposed between the side gears and the differential casing to allow the engine to provide torque to the wheel having traction. This initial torque transfer induces gear separating forces on the side gears, which forces tend to separate the side gears to further frictionally engage the clutch mechanism with the casing. The increased frictional engagement of the clutch allows more torque to be developed, thus further increasing the side gear separating forces and limiting the slippage between the side gears and the differential casing.
However, such preloaded clutches are usually always engaged, and thus are susceptible to wear, causing undesirable repair and replacement costs. Additionally, such clutch mechanisms usually employ spring mechanisms which add to the cost and difficulty of manufacture.
Additionally, such a preloaded clutch mechanism may lock the output shafts together in situations where differential rotation is necessary. For example, if the vehicle is making a turn when the wheels are sufficiently engaged on the road surface and a sufficient amount of torque is developed, the differential will tend to lock up the output shafts due to the action of the side gear separating forces created by the developed torque. This may occur, for example, during turns on surfaces with a high coefficient of friction while under acceleration. In such a case, even though differential rotation is required, the torque and side gear separating forces lock up the two output shafts causing one wheel to drag and slide along the road surface. This problem is evident in rear drive vehicles during turns under acceleration as the portion of the vehicle near the dragging wheel may tend to bounce up and down.
Another method of limiting slippage involves engaging a frictional clutch mechanism between the side gears and the differential casing based on the difference in rotational speeds between the two output shafts. Limited slip differentials employing this method are classified as speed-sensitive differentials. The frictional clutch may be actuated by various hydraulic pump mechanisms which may be external to the differential casing or may be constructed of elements disposed inside the differential casing. However, such mechanisms usually are complicated and also add to the cost and difficulty of manufacture. Further, speed sensitive differentials are "reactive", i.e., they react after a wheel has already lost traction.
A prior art method of limiting slippage involves using a flyweight governor in combination with a clutch mechanism wherein the governor actuates the clutch mechanism when a predetermined differential rotation rate is detected. However, prior art devices using such arrangements are configured such that the governor almost instantaneously applies extremely high clutch torque to the output shafts, essentially locking the two output shafts together. Applying locking torque in such a manner applies very high stresses on the output shafts and may result in fracturing the output shafts.
The above described methods actuate a clutch mechanism using mechanical or hydraulic arrangements. It is desirable to control the actuation of a limited slip feature using electronic control methods. Electronic control methods provide the advantages of accurate, reliable control within a narrow control band. Electronic control methods also allow operating parameters to be easily changed, for example by programming the electronic control systems to respond to a particular range of differentiation speeds or some other vehicle parameter such as throttle position.
In U.S. patent application Ser. No. 09/030,602, entitled "Electronically Controllable Limited Slip Differential", also assigned to the assignee of the present invention, several embodiments of electronically controllable differentials are disclosed which have a clutch mechanism for transferring torque between a differential casing and a side gear disposed therein upon the application of an initiating force by an electronic actuator. In the disclosed embodiments, the clutch mechanism comprises a cone clutch element and an insert disposed between the side gear and the rotatable casing, wherein the cone clutch element and a complementary frustoconical engagement surface provided on either the interior of the rotating differential casing or on an insert rotatably fixed to the casing. The cone clutch element and the side gear include camming portions having ramp surfaces which interact to produce axial movement of the cone clutch element with respect to the side gear when the initiating force is applied by the electronic actuator. The electronic actuator comprises an electronic control system having sensors which sense a predetermined rotational condition of the side gear and/or other selected components of the differential, and an electromagnet which applies the initiating force, which may be variable, to the cone clutch element.
During non-slipping conditions, the present controllable differential operates as an open differential wherein the cone clutch element is disengaged from the casing and rotates with the associated side gear. When a predetermined rotational condition of the differential components is sensed, such as relative rotation of the side gear with respect to the differential casing or relative rotation of the side gears and the pinion gears in excess of a predetermined level, the electronic control system actuates the electromagnet to apply an initiating force to the cone clutch element. The initiating force produces an initial axial movement of the cone clutch element such that the cone clutch element frictionally contacts the complementary frustoconical surface of the casing or insert and momentarily slows the cone clutch element down with respect to the side gear. The momentary slowdown causes the cam portions to interact and to provide axial separation forces which axially move the cone clutch element to thereby transfer a predetermined amount of torque from the rotatable casing to the side gear.
Electric current passes through the annular electromagnet comprising part of the electronic control system for actuating the clutch, and induces a magnetic field wherein the magnetic flux flows in a first direction through portions of the differential casing, clutch element, and other ferrous components. Continued exposure of these parts to the unidirectional magnetic flux causes the parts to become permanently magnetized, and causes metal debris within the differential to become attracted to the magnetized parts. This tends to result in wear of the differential components, such as the clutch surfaces, for example. The magnetizing flux which causes the parts to become permanently magnetized is called remanence flux. Remanence is the flux density remaining in magnetic material when the applied magnetic field strength is reduced to zero. Generally, the parts which carry the magnetic field are made from materials which have minimal remanence characteristics. However, many parts located in the vicinity of the flux path are made from materials which have undesirable remanence characteristics. This is especially true of hardened steel and its alloys which are used for the gears, bearings, and shafts in differentials.
The axle housing, within which the differential case is disposed, is ordinarily provided with a drain plug comprising a permanent magnet to collect the metal debris, so that the debris can easily be removed during ordinary maintenance procedures, such as differential oil changes. However, the debris may instead adhere to the surfaces of the permanently magnetized differential parts. Further, the debris may adhere to relatively moving, interactive surfaces, for example, bearing and clutch surfaces, causing accelerated wear thereof, which may result in degraded performance and failure of the differential. It must be noted that this remanence flux phenomenon and the metal debris accumulation problem associated therewith is not limited to electronically controllable differentials. Rather, this problem is common to all types of differentials, transmissions, transfer cases and similar mechanisms which employ electromagnets.
Thus, it is desirable to provide a system of demagnetizing differential components to prevent the magnetic adherence of metal debris to surfaces thereof.